Influence of various irradiation processes on the mechanical properties and polymerisation kinetics of bulk-fill resin based composites

ObjectivesTo assess the effect of irradiation time and distance of the light tip on the micro-mechanical properties and polymerisation kinetics of two bulk-fill resin-based composites at simulated clinically relevant filling depth.MethodsMicro-mechanical properties (Vickers hardness (HV), depth of cure (DOC) and indentation modulus (E)) and polymerisation kinetics (real-time increase of degree of cure (DC)) of two bulk-fill resin-based composites (Tetric EvoCeram^® Bulk Fill, Ivoclar Vivadent and x-tra base, Voco) were assessed at varying depth (0.1–6 mm in 100 μm steps for E and HV and 0.1, 2, 4 and 6 mm for DC), irradiation time (10, 20 or 40 s, Elipar Freelight2) and distances from the light tip (0 and 7 mm). Curing unit's irradiance was monitored in 1 mm steps at distances up to 10 mm away from the light tip on a laboratory-grade spectrometer.ResultsMultivariate analysis (α = 0.05), Student's t-test and Pearson correlation analysis were considered. The influence of material on the measured mechanical properties was significant (η² = 0.080 for E and 0.256 for HV), while the parameters irradiation time, distance from the light tip and depth emphasise a stronger influence on Tetric EvoCeram^® Bulk Fill. The polymerisation kinetics could be described by an exponential sum function, distinguishing between the gel and the glass phase. The above mentioned parameters strongly influenced the start of polymerisation (gel phase), and were of less importance for the glass phase.ConclusionsBoth materials enable at least 4 mm thick increments to be cured in one step under clinically relevant curing conditions.Clinical significanceThe susceptibility to variation in irradiance was material dependent, thus properties measured under clinically simulated curing conditions might vary to a different extent from those measured under ideal curing conditions.     

Effect of the degree of conversion of resin-based composites on cytotoxicity,cell attachment,and gene expression

ObjectiveThis study investigated the influence of the degree of conversion (DC), resin-based composites (RBC) composition, and the effect of additional violet light from one light curing unit (LCU) on cell attachment/growth, eluate cytotoxicity, and gene expression.MethodsThe effect of different DC of RBCs on human gingival fibroblasts (HGFs) when cultured directly onto cured RBCs, and when exposed afterwards to eluates in cell culture medium was examined. Venus^® (RBC-V; Bis-GMA-based) and Venus Pearl^® (RBC-P; TCD-DI-HEA and UDMA-based) were cured using a single emission peak (blue) light, Translux Wave^®; TW and a dual emission peak (blue-violet) light, Translux 2 Wave^®; T2W. To determine the value of the additional violet light from the T2W, exposure times and distances were adjusted to deliver similar radiant exposures (RE) from the blue region of both lights at five different RE levels from 1.5 J/cm² to 28.9 J/cm².ResultsBoth RBCs light-cured with the T2W at higher REs resulted in higher DC, increased cell adhesion and decreased eluate cytotoxicity. RBC-V induced greater cell adhesion, lower mRNA levels of pro-inflammatory markers, and higher mRNA levels of a proliferation marker than RBC-P. Wettability was the same for both RBCs. Toxicity decreased with increasing number of elution cycles. The initial eluates from RBC-P had a lower toxicity than from RBC-V.SignificanceRBCs cured with T2W (delivering both blue and violet light) at higher RE had greater DCs. The greatest DC and the least cell reactions were observed when the RE was >25 J/cm².     

Cuspal deflection,strain and microleakage of endodontically treated premolar teeth restored with direct resin composites

Objectives

To measure cuspal deflection and tooth strain, plus marginal leakage and gap formation caused by polymerization shrinkage during direct resin composite restoration of root-filled premolars.

Methods

Thirty-two first and second maxillary premolars were divided into four groups (n = 8). Group 1 had standardised mesio-occlusal-distal (MOD) cavities and served as the control group. Group 2 had endodontic access and root canal treatment through the occlusal floor of the MOD cavity, leaving the axial dentine intact. Group 3 had endodontic access and root canal treatment with the mesial and distal axial dentine removed. Group 4 had endodontic access and root canal treatment with axial dentine removed and a glass ionomer base (GIC). All groups were restored incrementally using a low shrink resin composite. Cuspal deflection was measured using direct current differential transformers (DCDTs), and buccal and palatal strain was measured using strain gauges. Teeth were immersed in 2% methylene blue for 24 h, sectioned and scored for leakage and gap formation under light and scanning electron microscopy.

Results

Total cuspal deflection was 4.9 ± 1.3 μm for the MOD cavity (group 1), 7.8 ± 3.3 μm for endodontic access with intact axial dentine (group 2), 12.2 ± 2.6 μm for endodontic access without axial dentine (group 3), and 11.1 ± 3.8 μm for endodontic access with a GIC base (group 4). Maximum buccal strain was 134 ± 56, 139 ± 61, 251 ± 125, and 183 ± 63 μstrain for groups 1–4 respectively, while the maximum palatal strain was 256 ± 215, 184 ± 149, 561 ± 123, 264 ± 87 μstrain respectively. All groups showed marginal leakage; however placement of GIC base significantly improved the seal (p = 0.007).

Conclusion

Cusp deflection and strain increased significantly when axial dentine was removed as part of the endodontic access. Placement of a glass ionomer base significantly reduced tooth strain and marginal leakage. Therefore, a conservative endodontic access and placement of a glass ionomer base are recommended if endodontically treated teeth undergo direct restoration with resin composite.     

Influence of radiant exposure on contraction stress, degree of conversion and mechanical properties of resin composites.

OBJECTIVE: To verify the influence of radiant exposure (H) on contraction stress (CS), degree of conversion (DC) and mechanical properties of two restorative composites. METHODS: Filtek Z250 (3M ESPE) and Heliomolar (Ivoclar) were photoactivated with 6, 12, 24, or 36 J/cm2 at continuous irradiance of 600 mW/cm2. CS at 10 min was determined in a low compliance testing system. DC, flexural strength (FS), flexural modulus (FM) and Knoop microhardness (KHN) were measured after 24 h storage at 37 degrees C. KHN and DC measurements were conducted on the irradiated surface of 1mm thick disk-shaped specimens. Bar-shaped specimens were submitted to three-point bending to determine FS and FM. Data were analyzed by one-way ANOVA/Tukey's test (alpha = 0.05) separately for each composite. RESULTS: For Filtek Z250, no significant increase in CS was observed above 12 J/cm2. DC and FM were similar at all H values, while FS increased significantly between 6 and 24 J/cm2. KHN was significantly different among all H levels, except between 12 and 24 J/cm2. For Heliomolar, CS and KHN increased significantly with H, except between 24 and 36 J/cm2. DC, FM and FS did not vary, regardless of the radiant exposure. SIGNIFICANCE: Variables tested behaved differently. CS and KHN were more sensitive to increasing radiant exposures than the other properties evaluated. FS varied only for Filtek Z250, while, for both composites, DC and FM were not affected by different H values.     

The use of an elastomeric methacrylate monomer (Exothane 24) to reduce the polymerization shrinkage stress and improve the two-body wear resistance of bulk fill composites

ObjectivesCharacterize the chemical structure of an elastomeric monomer (Exothane 24) and evaluate the degree of conversion (DC), polymerization shrinkage stress (PSS), rate of polymerization (Rp), flexural strength (F_Strenght), flexural modulus (F_Modulus), Vickers hardness (V_Hardness) and two-body wear resistance of dental bulk fill composites (BFCs) containing Exothane 24.MethodsThe Exothane 24 was characterized using mass spectroscopy, elemental analysis, ¹³C- and ¹H NMR. BFCs were formulated containing Exothane 24 (E10, E25, and E50). Similar BFCs containing regular UDMA (U10, U25, and U50), commercial conventional, and BFCs were used as control groups. ATR-FTIR spectroscopy was used to measure DC and the Rp of the composites. The PSS was measured using the universal testing machine method. Specimen bars were used to assess the F_Strenght, F_Modulus, and V_Hardness. RBCs were submitted to a two-body wear test using a chewing simulator machine; the rate and volumetric wear loss were evaluated using an optical scanner. Data were analyzed statistically with α = 0.05 and β = 0.2.ResultsExothane 24 is a urethane isophorone tetramethyl methacrylate monomer with polymerization stress-relieving properties. No differences were found in the DC up to 4 mm in depth for E25. All BFCs had similar F_Strenght, except for E50. E25 had the lowest volumetric wear loss and wear rate. E25 had approximately 30% lower PSS and slower Rp than commercial BFCs with similar wear resistance to conventional commercial composites.SignificanceThe Exothane 24 reduced the PSS and increased the wear resistance of BFCs; however, the formulation is important to optimize the properties of the BFCs.     

Influence of irradiant energy on degree of conversion, polymerization rate and shrinkage stress in an experimental resin composite system

OBJECTIVE: This study evaluated the degree of conversion (DC), maximum rate of cure (Rpmax), and polymerization stress (PS) developed by an experimental dental composite subjected to different irradiant energies (3, 6, 12, 24, or 48 J/cm2) under constant irradiance (500 mW/cm2). METHODS: DC and Rpmax were monitored for 10 min on the bottom surface of 2-mm thick disks and on 150-microm thick films (representing the top of the specimen) using ATR-FTIR. PS was monitored for 10 min in 2-mm thick disks bonded to two glass rods (?=5mm) attached to a universal testing machine. One-way ANOVA/Tukey tests were used and differences in DC and R(p)(max) between top and bottom surfaces were examined using Student's t-test. Statistical testing was performed at a pre-set alpha of 0.05. RESULTS: For a given surface, DC showed differences among all groups, except at the top between 24 and 48 J/cm2. Rpmax was similar among all groups at the same surface and statistically higher at the top surface. PS also showed significant differences among all groups. Data for 48 J/cm2 were not obtained due to specimen failure at the glass/composite interface. SIGNIFICANCE: Increases in irradiant exposure led to significant increases in DC and PS, but had no effect on Rpmax.     

Contraction stress, elastic modulus, and degree of conversion of three flowable composites

The aim of this study was to measure the contraction stress of three flowable resin composites and to correlate the stress with the elastic modulus and the degree of conversion. One low-shrinkage (Venus Diamond Flow) and two conventional (Tetric EvoFlow and X-Flow) flowable composites were polymerized for 40s with a light-emitting diode (LED) curing unit. Contraction force was continuously recorded for 300s using a stress-analyser, and stress values were calculated at 40s and at 300s. The maximum stress rate was also calculated for each specimen. The elastic modulus of each composite was assayed using a biaxial flexural test, and degree of conversion was analysed with Raman spectroscopy. X-Flow exhibited higher stress values than the other tested materials. Venus Diamond Flow showed the lowest stress values at 40s and at 300s, and the lowest maximum stress rate. Stress values were correlated with elastic modulus but not with degree of conversion, which was comparable among all tested materials.     

Objects build orientation,positioning, and curing influence dimensional accuracy and flexural properties of stereolithographically printed resin

Objective

To evaluate the influence of printing parameters on flexural properties and accuracy of SLA-printed standard objects.

Simultaneous measurement of polymerization stress and curing kinetics for photo-polymerized composites with high filler contents

ObjectivesPhotopolymerized composites are used in a broad range of applications with their performance largely directed by reaction kinetics and contraction accompanying polymerization. The present study was to demonstrate an instrument capable of simultaneously collecting multiple kinetics parameters for a wide range of photopolymerizable systems: degree of conversion (DC), reaction exotherm, and polymerization stress (PS).MethodsOur system consisted of a cantilever beam-based instrument (tensometer) that has been optimized to capture a large range of stress generated by lightly-filled to highly-filled composites. The sample configuration allows the tensometer to be coupled to a fast near infrared (NIR) spectrometer collecting spectra in transmission mode.ResultsUsing our instrument design, simultaneous measurements of PS and DC are performed, for the first time, on a commercial composite with ≈80% (by mass) silica particle fillers. The in situ NIR spectrometer collects more than 10 spectra per second, allowing for thorough characterization of reaction kinetics. With increased instrument sensitivity coupled with the ability to collect real time reaction kinetics information, we show that the external constraint imposed by the cantilever beam during polymerization could affect the rate of cure and final degree of polymerization.SignificanceThe present simultaneous measurement technique is expected to provide new insights into kinetics and property relationships for photopolymerized composites with high filler content such as dental restorative composites.     

The effect of high-irradiance rapid polymerization on degree of conversion,monomer elution,polymerization shrinkage and porosity of bulk-fill resin composites

ObjectiveThe purpose was to compare the degree of conversion (DC), monomer elution (ME), polymerization shrinkage (PS) and porosity of two addition-fragmentation chain transfer (AFCT) modified resin-based composites (RBC) light-cured with rapid- (RP), turbo- (TP) or conventional polymerization (CP) settings.MethodsCylindrical samples (6-mm wide, 4-mm thick) were prepared from Tetric PowerFill (TPF) and Filtek One Bulk (FOB). Four groups were established according to the polymerization settings: 3s-RP, 5s-TP, 10s-CP and 20s-CP. Samples in 1 mm thickness with 20s-CP settings served as controls. The DC at the top and bottom surfaces was measured with micro-Raman spectroscopy. ME was detected with high-performance liquid chromatography. PS and porosity were analyzed by micro-computed tomography. ANOVA and Tukey’s post-hoc test, multivariate analysis and partial eta-squared statistics were used to analyze the data (p < 0.05).ResultsFOB showed higher DC values (61.5–77.5 %) at the top compared to TPF (43.5–67.8 %). At the bottom TPF samples achieved higher DCs (39.9–58.5 %) than FOB (18.21–66.18 %). Extending the curing time increased DC (except the top of FOB) and decreased ME. BisGMA release was the highest among the detected monomers from both RBCs. The amount was three-fold more from TPF. The factor Material and Exposure significantly influenced DC and ME. PS (1.8–2.5 %) did not differ among the groups and RBCs except for the lowest value of TPF cured with the 3s_RP setting (p = 0.03). FOB showed 4.5-fold lower porosity (p < 0.001). Significantly higher pore volume was detected after polymerization in 3s_RP (p < 0.001).SignificanceHigh-irradiance rapid 3-s curing of AFCT modified RBCs resulted in inferior results for some important material properties. A longer exposure time is recommended in a clinical situation.     

Influence of photoinitiator type on the rate of polymerization, degree of conversion, hardness and yellowing of dental resin composites

OBJECTIVES: To evaluate the degree of conversion (DC), maximum rate of polymerization (Rpmax), Knoop hardness (KHN) and yellowing (b-value) of resin composites formulated with phenylpropanedione (PPD), camphorquinone (CQ), or CQ/PPD at different concentrations. The hypotheses tested were (i) PPD or CQ/PPD would produce less Rpmax and yellowing than CQ alone without affecting DC and KHN, and (ii) Rpmax, DC, and KHN would be directly related to the absorbed power density (PDabs). METHODS: CQ/amine, PPD/amine and CQ/PPD/amine were used at low, intermediate and high concentrations in experimental composites. Photoinitiator absorption and halogen-light emission were measured using a spectrophotometer, Rp with differential scanning calorimetry (DSC), DC with DSC and FTIR, KHN with Knoop indentation; and color with a chromameter. The results were analyzed with two-way analysis of variance (ANOVA)/Student-Newman-Keul's test (p<0.05). Correlation tests were carried out between PDabs and each of DC, Rpmax and KHN. RESULTS: The PDabs increased with photoinitiator concentration and PPD samples had the lowest values. In general, maximum DC was comparable at intermediate concentration, while Rpmax and KHN required higher concentrations. DC was similar for all photoinitiators, but Rpmax was lower with PPD and CQ/PPD. PPD produced the lowest KHN. Yellowing increased with photoinitiator concentration. PPD did not reduce yellowing at intermediate and/or high concentrations, compared to CQ-formulations. PDabs showed significant correlations with DC, Rpmax and KHN. CONCLUSION: PPD or CQ/PPD reduced Rpmax in experimental composites without affecting the DC. The use of PPD did not reduce yellowing, but reduced KHN. DC, Rpmax and KHN were dependent on PDabs.     

Effect of calcium orthophosphate particle size and CaP:glass ratio on optical,mechanical and physicochemical characteristics of experimental composites

ObjectiveEvaluate light transmittance (%T), color change (ΔE), degree of conversion (DC), bottom-to-top Knoop microhardness (KHN), flexural strength (BFS) and modulus (FM), water sorption/solubility (WS/SL) and calcium release of resin composites containing different dicalcium phosphate dihydrate (DCPD)-to-barium glass ratios (DCPD:BG) and DCPD particle sizes.MethodsTen resin-based composites (50 vol% inorganic fraction) were prepared using BG (0.4 µm) and DCPD particles (12 µm, 3 µm or mixture) with DCPD:BG of 1:3, 1:1 or 3:1. A composite without DCPD was used as a control. DC, KHN, %T and ΔE were determined in 2-mm thick specimens. BFS and FM were determined after 24 h. WS/SL was determined after 7 d. Calcium release was determined by coupled plasma optical emission spectroscopy. Data were analyzed by ANOVA/Tukey test (alpha: 0.05).Results%T was significantly reduced in composites with milled, compared to pristine DCPD (p < 0.001). ΔE > 3.3 were observed with DCPD:BG of 1:1 and 3:1 formulated with milled DCPD (p < 0.001). DC increased at 1:1 and 3:1 DCPD:BG (p < 0.001). All composites presented bottom-to-top KHN of at least 0.8. BFS was not affected by DCPD size but was strongly dependent on DCPD:BG (p < 0.001). Reductions in FM were observed with milled DCPD (p < 0.001). WS/SL increased with DCPD:BG (p < 0.001). At 3DCPD: 1BG, using small DCPD particles led to a 35 % increase in calcium release (p < 0.001).SignificanceA trade-off between strength and Ca²⁺ release was observed. In spite of its low strength, the formulation containing 3 DCPD: 1 glass and milled DCPD particles is preferred due to its superior Ca²⁺ release.     

Synthesis and study of properties of dental resin composites with different nanosilica particles size

Objectives

The aim of this work was the synthesis of light-cured resin nanocomposites using nanosilica particles with different particle size and the study of some physical-mechanical properties of the composites.

Methods

Various types of silica nanoparticles (Aerosil) with average particle size of 40, 20, 16, 14, and 7 nm, used as filler were silanized with the silane 3-methacryloxypropyl-trimethoxysilane (MPS). The total amount of silane used was kept constant at 10 wt% relative to the filler weight to ensure the complete silanization of nanoparticles. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (55 wt%) were mixed with a photoactivated Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of composites was determined by FT-IR analysis. The static flexural strength and flexural modulus were measured using a three-point bending set up. The dynamic thermomechanical properties were determined by dynamic mechanical analyzer (DMA). Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water solution 75 vol% for 30 days. The TGA for composites was performed in nitrogen atmosphere from 30 to 700 °C.

Results

As the average silica particle size decreases, the percentage amount of MPS attached on the silica surface increases. However, the number of MPS molecules attached on the silica surface area of 1 nm² is independent of filler particle size. As the average filler particles size decreases a progressive increase in the degree of conversion of composites and an increase in the amount of sorbed water is observed.

Significance

The prepared composites containing different amount of silica filler, with different particle size, but with the same amount of silanized silica and organic matrix showed similar flexural strength and flexural modulus, except composite with the lowest filler particle size, which showed lower flexural modulus.     

Effect of thiourethane filler surface functionalization on stress,conversion and mechanical properties of restorative dental composites

Objectives

This study evaluated the efficacy of a thiourethane(TU)-modified silane agent in improving properties in filled composites.

Development of dual-cured resin cements with long working time,high conversion in absence of light and reduced polymerization stress

ObjectiveThis study evaluated the properties of experimental dual-cured cements containing thiourethane (TU) and low concentrations of p-Tolyldiethanolamnie (DHEPT) and benzoyl peroxide (BPO) as chemical initiators.MethodsBisGMA/TEGDMA-based dual-cured cement was formulated with 1.0 wt% DHEPT and 0.75 wt% BPO as initiators and used as control. The concentration of BPO was adjusted to 0.1 wt% in catalyst paste of experimental cements, and two base pastes containing TU and 0.5 wt% or 0.25 wt% of DHEPT were formulated. The rheological behavior and kinetics of polymerization of cements were assessed in the absence of light activation. The kinetics of polymerization was also evaluated for cements light-activated immediately or 5 min after the start of mixing. Polymerization stress, flexural strength and elastic modulus (n = 5) were also evaluated under these conditions.ResultsCements with TU presented lower viscosity than the control, improved working time (0.25% DHEPT > 0.5% DHEPT) and higher conversion in the absence of light-activation. Delaying the light-activation reduced the maximum rate of polymerization (Rp_max) but did not affect the conversion or stress. The addition of TU increased the Rp_max and conversion, and reduced the stress when compared to the control, without affecting the flexural strength. Except for the control with delayed light-activation (highest values), the other experimental conditions yielded similar modulus.SignificanceAdding TU and using a low concentration of DHEPT/BPO resulted in dual-cured cements with longer working time, reduced polymerization stress and increased conversion even in the absence of light, with no significant effect on the mechanical properties.